Positioning systems are used for the accurate and reproducible positioning of patients and equipment for radiation therapy, diagnostic imaging, surgery and other medical procedures (hereinafter all these are referred to collectively as “radiotherapy treatments”). For modern radiotherapy treatments, precise positioning is imperative; accurate positioning is not only necessary for the patient, but also for accessories, such as supports for the patient or parts of the patient's body (e.g. body bags or cushions, headsteps, neck supports, kneesteps, etc.), or other items of equipment which are employed during radiotherapy. Positioning is normally relative to the room or facility in which the radiotherapy is to take place, in particular relative to the treatment apparatus itself (the apparatus which generates the radiation for the radiotherapeutic treatment). This apparatus is usually large and heavy, so the patient and/or accessories are usually placed on a support, or table, which is moveable and has at least three translational degrees of freedom and often additional rotational degrees of freedom. In many cases, once the patient, and any accessories, are correctly positioned relative to the radiation apparatus (for example by moving the table on which the patient is supported), the radiotherapy apparatus moves relative to the patient during the radiotherapy treatment so as to direct a beam of radiation towards the patient from a number of different directions, to irradiate a desired region sufficiently whilst reducing the radiation dose applied to healthy tissue lying around the desired region; this is usually done by mounting the radiation source on a gantry which is free to rotate in an arc around the patient, thus allowing the radiation beam to be precisely directed and the radiation dose to be accurately controlled. These movements are typically controlled by patient and gantry control computers, and a radiation control computer controls the radiation generation, all according to a treatment planning computer; these may physically be one or more computers, or they may be discrete functional elements of a single computer.
Radiotherapy treatments are often set out in complex treatment plans, requiring the patient to undergo repeated radiotherapy treatments at intervals which can span a considerable time. This necessitates ensuring, every time that the patient attends for radiotherapy treatment, not only that the patient and any accessories or other equipment are accurately positioned, but also that the patient and the accessories or other equipment are correctly identified and match the treatment plan. The treatment plan is usually defined by a consultant physician, and translated into a plan which is capable of being carried out by the apparatus by a radiologist, with this plan being held in the treatment planning computer. When a patient attends for radiotherapy treatment, an operator calls up the appropriate treatment plan for that patient, ensures that the patient and all the necessary accessories/other items of equipment are present, match the treatment plan and are correctly positioned. This process of preparing the patient for the radiotherapy treatment (or “set up”) is complicated and takes time to carry out correctly, but is absolutely necessary to ensure patient safety and that the radiotherapy treatment is properly and effectively carried out. Reducing this set up time will improve the efficacy and safety of treatment and increase patient throughput, and many workflow management systems have been suggested to achieve this, often integrated with the treatment planning computer.
Systems employing radiofrequency identification (RFID) tags have been used for identification of the patient and any accessories or other equipments, but these are not yet sufficiently accurate to be used as the sole means of patient positioning (RFID tags are currently accurate to about 1 cm, whereas the radiotherapy treatment requires greater precision, of 1 mm or less). Light or laser projectors are used in nearly all radiotherapy treatment facilities to assist in positioning for radiotherapy treatment; beams of light are projected to form crosses on the accessory/skin of the patient. The projections are aligned with a predetermined area on the accessory/patient, which may be denoted by marks or stickers applied to the accessory/patient's skin. Photogrammetry-based systems have been used for positioning the patient, in which optically reflective markers affixed to the patient are tracked by infrared cameras, so that the accessory/patient can be accurately positioned initially (assuming that the marker has been accurately positioned on the patient's skin), and such systems are able to monitor any movement of the patient with sufficient accuracy during treatment, and sound an alert or interrupt the treatment in the event the movement exceeds a threshold which would seriously affect the radiotherapy treatment or risk harm to the patient.